Known in the past has been an exhaust purification system of an internal combustion engine which provides an exhaust purification catalyst which is provided in an exhaust passage of the internal combustion engine, an air-fuel ratio sensor at an upstream side of the exhaust passage in the direction of flow of exhaust, and an oxygen sensor at a downstream side in the direction of flow of exhaust. Such an exhaust purification system, for example, performs feedback control so that the output of this air-fuel ratio sensor becomes a target value corresponding to a target air-fuel ratio based on the output of the upstream side air-fuel ratio sensor and uses the output of the downstream side oxygen sensor as the basis to correct the target value of the upstream side air-fuel ratio sensor.
In the exhaust purification system which is described in JP2011-069337A, when the output voltage of the downstream side oxygen sensor is the high side threshold value or more and the state of the exhaust purification catalyst is an oxygen deficient state, the target air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is made a lean air-fuel ratio. Conversely, when the output voltage of the downstream side oxygen sensor is the low side threshold value or less and the state of the exhaust purification catalyst is an oxygen excess state, the target air-fuel ratio is made a rich air-fuel ratio. According to the exhaust purification system which is described in JP2011-069337A, due to this, when in the oxygen deficient state or oxygen excess state, it is considered possible to quickly return the state of the exhaust purification catalyst to an intermediate state of these two states (that is, the state where the exhaust purification catalyst stores a suitable amount of oxygen).